1. Field of the Invention
The present invention relates to a step linear actuator which effects linear motion using on electromechanical conversion means.
2. Description of the Prior Art
Hitherto there has generally been available a method of converting a rotary motion into a linear motion using a conversion mechanism such as a ball screw, rack and pinion, etc. For the motor a rotary motion, as shown in the optical pickup traverse means of an optical disk drive for reproducing recorded information from a disk type recording medium having, for example, a spirally recorded information track.
Futhermore, as a means of directly driving a driven member without using the above conversion mechanism, a linear motor or a linear pulse motor is known. These driving apparatuses are composed of a stator comprising mainly magnetic materials and forming a traveling path and a mover disposed opposite to said stator in a slight gap.
For example, an seen in the case of the linear pulse motor (L. Tsai; Design provides High Plotter performance at Low Cost H. P. Journal 1979-2), the above stator is composed of magnetic pole teeth of a certain pitch, and the mover is composed of a pair of yokes having different pitches from that of the magnetic pole teeth of the stator and holding a permanent magnet therebetween and a pair of the electromagnetic coils wound on the yokes. The mover is displaced stepwise to the stator by alternate excitation of the electromagnetic coils.
As such, as a means of linear driving, the method of using a transform mechanism or the method of direct drive has so far been known. Either of these methods has complicated construction of traveling path and requires a large occupying area, so it has a limit in obtaining an effective stroke.
According to these constructions, because the mover is caused to travel via a certain small gap from the stator, high accuracy is demanded of both the whole stator and the mover, leading to an increase in the production cost.
Futhermore, the suspension of the mover makes it difficult to keep stability to the traveling path due to the vibration and impact exerted from the exterior of the apparatus and the positional condition.
On the other hand, there has been proposed a method as shown in U.S. Pat. No. 4,697,164 as another direct drive different from the direct drive as in the above linear motor. This construction comprises a pair of stators coaxially mounted on a shaft and spaced from each other in the axial direction of the shaft, a pair of armatures freely slidably supported by the shaft relative to said stators, a pair of latching means each having a center opening surrounding said shaft and being rotatably connected to said armatures at its end, and a spring means disposed between said stators and the armatures for urging the armatures away from the stators. By alternately exciting a pair of electromagnetic coils contained in the stators, the armatures are attracted to the stator side against the pressing force of the spring means, and, in collaboration with said armatures, the latching means on the disk performs pivotal movement at the fulcrum of pivotal movement at its end, so that the shaft is gripped with said center opening, by which linear movement is performed.
This construction necessitates a mechanical grip means besides the electromagnetically operating means, requiring a large number of components with increase in size and cost, and further, due to mechanical grip of the shaft, wear occurs, thereby providing problems in reliability and precision of the apparatus.
Further, according to this construction, because the armatures are at all times attracted to the stator side against the pressing force of the spring means, the energy to be applied becomes large in obtaining an effective driving force against the driven member.